Reversibly variable electrochromic devices have been previously described. In such devices, the intensity of light (e.g., visible, infrared, ultraviolet or other distinct or overlapping electromagnetic radiation) is modulated by passing the light through an electrochromic medium. The electrochromic medium is disposed between two conductive electrodes, at least one of which is typically transparent, which causes the medium to undergo reversible electrochemical reactions when potential differences are applied across the two electrodes.
Electrochromic materials comprise redox-active species that can exhibit significant, lasting, and reversible changes in color upon the injection or withdrawal of electrons. Electrochemical manipulation of the redox processes in thin layers of these materials thus allows the modulation of the spectral characteristics of light that is transmitted. Upon oxidation, for instance, an electrochromic material can switch from its clear and transparent form to a colored form, or vice versa.
Devices utilizing electrochromic materials are lauded for their low power consumption due to the passive transmissive mechanism of their operation, as opposed to the active emissive mechanism associated with light emitting diodes (LEDs). Specifically, electrochromic materials require only an initial potential pulse sufficiently strong to induce the desired electrochemical reaction and accompanying chromic change; in the absence of oxidizing or reducing contaminants, this change is permanent and the device may only demand backlighting. LEDs, on the other hand, require a constant power supply for continuous photoemission. Additionally, electrochromic devices are easy to fabricate as the electrochromic materials can be coated directly onto transparent electrodes without the need for the complex patterning required for LEDs.
Electrochromism has been reported in redox-active inorganics, such as tungsten oxide, small molecule organics, such as viologen, as well as conducting polymers, such as copolymers and derivatives of polypyrrole and polythiophene. Although tungsten oxide was the first electrochromic material of commercial interest, inorganic electrochromic materials typically exhibit slow switching speeds (101-102 seconds) and have proven costly to process due to the need for high-vacuum sputtering deposition. Some small-molecule organics, though easy to process, have shown poor stability, as they easily diffuse away from the electrode and into the electrolyte.
Electrochromic polymers, alternatively, hold the promise of robust film integrity, facile film formability, and fast switching times (<1 second) between oxidation states. P-type conducting polymers, such as poly(3,4-ethylenedioxythiophene) (PEDOT) and polypyrroles, exhibit an oxidative doping mechanism that can be reversibly accessed via electrochemical processes. Oxidative doping facilitates delocalization of charge carries along the polymer backbone; the electronic excitations that accompany photoabsorption thus occur at larger wavelengths relative to those of undoped species. Oxidative doping thus results in a decrease in π-π* transitions in favor of lower energy polaron charge carrier transitions spectroscopically. Given that these transitions typically occur at energy levels within the visible spectrum, many conductive polymers possess intrinsic electrochromicity. PEDOT, for instance, exhibits reversible chromic shifts from dark blue (π-π* absorption near λ=600 nm) to virtually clear and transparent (polaron band near λ=900 nm) upon oxidation. Chemically modifying thiophene and pyrrole monomers may alter the spectral properties of their polymerized form significantly. Some derivatives may even possess multiple oxidation states, wherein the switching between oxidation states results in polyelectrochromic characteristics (i.e., multiple colored states accessible).
PANI has applications in the field of polymer electronics due to its varied polyelectrochromic effects and the unique origin of its conductive form. Unlike many other conductive polymers, such as polyacetylene and polythiophene, PANI relies on a nonredox protonic doping mechanism and is inherently more resilient to the oxidizing environment presented in ambient conditions. And unlike derivitives and copolymers of polythiophenes and polypyrroles, PANI is directly capable of multiple redox transitions without the need for chemical derivitization of the monomer.
In its fully oxidized and reduced states, PANI is electrically insulating and referred to as pernigraniline base (PB) and leucoemeraldine base (LB), respectively. The LB state is characteristically transparent (λmax=˜330 nm), while the PB state typically possesses a dark violet color (λmax=˜550 nm). An intermediate oxidation state containing an equal number of oxidized and reduced repeat units exists between these two extremes and is referred to as emeraldine base (EB).
The electrically conductive emeraldine salt (ES) results from exposing the emeraldine base (EB) to a proton source, which is typically green in color (ES; λmax=˜800 nm). Early studies using small-molecule proton sources such as hydrochloric acid, sulfuric acid and camphorsulfonic acid demonstrate that while stable switching between the green ES state and the transparent LB state can be performed on ˜1 second time and within a ±1V potential window in acidic media (<pH 3), such films lose electroactivity at a higher pH as the proton dopants are neutralized and the small-molecule counterions responsible for charge-balancing the doped ES state diffuse away. For example, while conductivities spanning 10-300 S·cm−1 have been observed, negligible solubility and diminished electrochemical redox behavior above pH 4 due to neutralization and subsequent dopant diffusion have also been observed. Additionally, the volatile nature of these small-molecule acids may lead to diminished conductivities upon long-term storage, compromising the stability attributed to the nonredox doping mechanism. Thus, polyelectrochromic switching, though theoretically possible with PANI, has not been practically achieved.
Several methods have been proposed to solubilize ES while retaining electroactivity (i.e., redox behavior) at elevated pH. Some methods propose the derivitization of the PANI itself; such as sulfonating the benzenoid rings to produce self-doped PANI obviates issues of dopant volatility and diffusivity while widening the electroactive range toward alkaline conditions. However, while such derivitizations can impart practical levels of solubility to the final product, the sulfonation reaction can hydrolyze PANI, thereby reducing the electrical conductivity of the resulting polymer as a consequence (≦10−1 S·cm−1). As a result, the use of polyanions such as poly(acrylic acid), poly(styrene sulfonate), and poly(2-acrylamido-2-methyl-1-propane-poly(2-acrylamido-2-methyl-1-propane-sulfonic acid) (PAAMPSA) as dopants has been investigated as an alternative. It is believed that polymer acids may offer advantages over conventional small-molecule acid dopants because, for example, not all acid groups along the polymer chain participate the doping process, such that excess acid groups render water dispersibility to the conductive form of PANI.
Accordingly, there exists a need to continue to develop PANI materials having an ES state that is soluble and retains electroactivity at an elevated pH. In particular, there exists a need to develop PANI films that can undergo stable and reversible transitions between the ES, LS, and PB oxidation states, wherein the stable transitions can take place at or near a neutral pH with fast switching speeds (e.g., 100-101). There is also a need to develop PANI films exhibiting transitions to the PB oxidation state without chemical degradation or decay in the optical response.